Single element light detector

ABSTRACT

A single element hemispherical light detector that employs the concept of constructed occlusion to improve its uniformity of detection response across a large range of incident angles, and that incorporates a deflector to provide detection response to low incidence angles. The hemispherical light detector uses one active element or photodiode to achieve a substantially uniform response within a sector of a hemisphere.

BACKGROUND OF THE INVENTION

The present invention relates generally to light detectors that arerelatively insensitive to the angle of incident light over apredetermined spherical sector and, more particularly, to lightdetectors that uniformly detect light from any direction in a hemisphereand provide a measurement of the light's intensity.

A planar photodiode is an example of a light detector that is sensitiveto the angle of incident light. The photodiode's planar or flat surfaceexhibits a maximum cross-sectional area to light that is incident at anangle normal to the flat surface. However, as the angle of incidentlight increases from the normal, the cross-sectional area of the flatsurface decreases as a function of the cosine of the angle. Accordingly,the planar photodiode's response is related to the angle of the incidentlight by the angle's cosine.

Further, photodiodes, like most photodetectors, suffer from anadditional effect that depends on the angle of the incident light calledFresnel reflection. Fresnel reflection generally occurs whenever lighttravels through a surface between two materials having different indicesof refraction, for example, air and glass or silicon. As the incidentlight angle increases from the normal, the Fresnel reflection alsoincreases, which decreases the amount of light actually entering thedetector.

Currently known hemispherical light detectors generally employ either atranslucent diffuser or a multi-element detector system. The translucentdiffuser is a sheet of translucent material placed over the photodiode'ssurface. Incident light passes through the translucent sheet and isdiffused over a large angle, a portion of which is intersected by thephotodiode. The translucent diffuser can be made of many materialsincluding ground glass, acrylics, or Teflon. However, the translucentdiffuser fails to eliminate the cosine effect discussed above and,because the photodiode intercepts only a portion of the diffused light,it is generally inefficient. Multi-element detectors employ severalphotodiode elements that are each configured to cover a predeterminedspherical sector. Coverage over a larger spherical sector is obtained bycombining the signals from the several detectors. While multi-elementdetector systems are capable of providing hemispherical coverage, theirreliance upon multiple elements dramatically increases the cost andcomplexity of the system. The effectiveness of the multi-elementdetector systems is also lessened due to non-uniformities which occurfrom transitions from one element to the next.

Accordingly, there is a need for a light detector which is relativelyinsensitive to the angle incidence of a light source in a relativelysimple and cost effective manner. The present invention satisfies thisneed.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention resides in a lightdetector that provides a response signal based on the intensity of lightincident from any direction within a predetermined sector. The lightdetector includes a base, mask and a sensor. The base has a surfaceformed of a diffusely reflective material that defines a reflectiveregion that faces the spherical sector. The mask is spaced apredetermined distance from the reflective region and is located betweenthe base and the predetermined sector such that, when light is incidentat an angle normal to the base's surface, the mask occludes a portion ofthe reflective region. The sensor is responsive to incident light andgenerates the response signal based on the intensity of light that itreceives and the sensor is located to intercept light reflections fromthe base surface. The base, mask and sensor are configured such that thelight detector is uniformly responsive to light from any directionwithin the predetermined sector.

In a more detailed feature of the invention, the reflective regionfurther includes a hemispherical cavity and the reflective region isdefined by the cavity's aperture. Further, the reflective region furtherincludes a shoulder that surrounds the cavity's aperture. Also, the maskis a circular disk having at least one flat surface and an axis throughthe flat surface's center. The disk's axis is aligned with an axisthrough the center of the hemispherical cavity such that the disk's flatsurface is parallel with the cavity's aperture.

In another detailed feature of the invention, the disk's diameter isabout 90% of the diameter of the hemispherical cavity. Also, the mask isspaced away from the aperture at a distance that is about 10% to 15% ofthe diameter of the hemispherical cavity. Further, the sensor is aphotodiode located between the mask and the reflective region, and itsresponse signal is an electrical signal.

In another detailed feature of the invention, the light detector furtherincludes a deflector that directs light incident from a direction nearlyparallel with the aperture toward the aperture. The light deflector maybe an orthogonal pair of baffles that are oriented to diffusely reflectlight incident from a direction nearly parallel with the aperture towardthe aperture and the photodiode. Also, the deflector may be formed froma disk having a thickness that is about 10% of the diameter of thehemispherical cavity. The disk's edge is beveled and reflective so thatit directs light incident from a direction nearly parallel with theaperture toward the aperture.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred embodiments,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first embodiment of ahemispherical light detector, in accordance with the present invention,that uniformly measures the intensity of light from any direction in ahemisphere using a single photodiode element.

FIG. 2 is a cross-sectional view showing selected elements of thehemispherical light detector of FIG. 1, to emphasize the geometricalrelationship between the photodiode element and a mask and a diffuselyreflective surface.

FIG. 3A is a schematic diagram of a mask and a photodiode's surface, inaccordance with the present invention, illuminated by light incident atan angle normal to the photodiode's surface.

FIG. 3B is a schematic diagram of a mask and a photodiode's surface, inaccordance with the present invention, illuminated by light incident atan angle of approximately 15 degrees from normal.

FIG. 3C is a schematic diagram of a mask and a photodiode's surface, inaccordance with the present invention, illuminated by light incident atan angle of approximately 75 degrees from normal.

FIG. 4A is a perspective view of a deflector, in accordance with thepresent invention, that diffusely reflects light incident at an anglenearly horizontal to the photodiode's surface.

FIG. 4B is a schematic diagram of a mask, a photodiode's surface, andthe deflector of FIG. 4A, illuminated by light incident at an angle ofapproximately 75 degrees from normal.

FIG. 5 is a schematic diagram of a mask and a photodiode's surface, inaccordance with the present invention, further including the deflectorof FIG. 4.

FIG. 6 is a cross-sectional view of a beveled mask, in accordance withthe present invention, having beveled sides that reflects light incidentat an angle nearly horizontal to the photodiode's surface.

FIG. 7 is a cross-sectional view of a second embodiment of ahemispherical light detector, in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawings, and in particular to FIG. 1, thereis shown a first embodiment of a hemispherical light detector 10 thatprovides an electrical signal based on the intensity of light incidentfrom any direction within a predetermined sector of a spherical or otherappropriate coordinate system. The light detector includes a photodiode12, a disk-shaped mask 14, a deflector 16, and a diffusely reflectivebase 18. The detector's geometrical configuration compensates for theangle or direction dependent response of a standard photodiode.

The base 18 is a disk-shaped block of material having, formed in itsupper flat surface 20, a hemispherical cavity 22 that is surrounded by aflat ring-shaped shoulder 24. A significant function of thehemispherical cavity is to provide a diffusely reflective surface thataverages the incoming light at the cavity's aperture and a cavity with ahemispherical shape is preferred because of its azimuthal symmetry andits ease in construction. However, other cavity shapes are acceptable.For purposes of describing the light detector's operation, a goodapproximation is obtained by treating the cavity as if it were adiffusely reflective flat surface that averages the incident light inthe plane of the cavity's aperture 26. Preferably, the base isconstructed of Spectralon®, which is a highly reflective polymeric blockmaterial manufactured and sold by Labsphere Inc., of North Sutton, N.H.Spectralon® is easily machined, very durable, and provides a highlyefficient Lambertion surface having a reflectivity of over 99%, innear-infrared and visible wavelengths. A Lambertian surface emits lightwith a substantially uniform intensity in all directions. Alternatively,the base could be constructed of a suitable base material of, forexample, aluminum or plastic with the reflective surfaces, i.e., thehemisphere and the shoulder, coated with a diffusely reflective materialsuch as barium sulfate or Spectralon®.

The photodiode 12 is a commercially available photodiode (PIN-25DP) soldby United Detector Technologies (UDT) Sensors, Inc., of Hawthorne,Calif. The photodiode is mounted in a protective can 28 having a frontwindow 30 and a rear-mounted BNC connector 32. In this embodiment, theprotective can also functions as the mask. The photodiode assembly isheld in place over the base 18 by a Pyrex® dome 34 that covers thebase's upper side. The photodiode generates an electrical currentgenerally proportional to the intensity of light incident on itssurface. The photodiode is connected to external measurement electronicsusing a standard BNC connector and appropriately gauged electrical wires36. Suitable measurement electronics can be readily obtained from avariety of electronic amplifier manufacturers.

The geometrical relationship between the can or mask 14, the photodiode12, and the aperture 26 formed by the base cavity 22 are shown in FIG.2. The ratio between the mask's diameter and the aperture's diameter,and the distance between the mask and the aperture, are the mostsignificant parameters in optimizing the light detector's responseuniformity and efficiency. The limits on the response uniformly definethe limits of the response sector. A more uniform response is obtained,or the response sector increases, as the mask/aperture diameter ratioapproaches one. However, the detector's sensitivity decreases as themask aperture/diameter ratio approaches one because the aperture'sacceptance area necessarily decreases. In the embodiment shown, themask's diameter is 1.68 inches and the aperture's diameter is 1.86inches, which results in a mask/aperture diameter ratio of approximately0.9 or 90%. A mask/aperture diameter ratio of 0.9 provides a relativelyuniform response and large response sector while maintaining anacceptable sensitivity. Further, the disk-shaped mask is spaced awayfrom the aperture by 0.19 inches resulting in a mask distance toaperture diameter ratio of approximately 0.1 or 10%.

With reference to FIGS. 3A-3C, the light detector 10 takes advantage ofa technique called constructed occlusion to reduce the cosine dependenceof a photodiode 12. In the occlusion technique, the mask's diameter isslightly less than the photodiode's diameter. When the light is incidentat an angle normal to the surfaces of the mask 14 and photodiode, onlythe incident light not intercepted by the mask reaches the photodiode,as indicated by the two regions, 38 and 40, each having a width of A_(S)/2 (FIG. 3A). The total cross-sectional width of the two regions isA_(S). As the angle of the incident light increases from the normaldirection, the cross-sectional width of the light in the first region 38decreases by A.sub.θ while the cross-sectional width of the light in thesecond region increases by A.sub.θ (FIG. 3B). Accordingly, as long as aportion of the mask's shadow remains on the photodiode, the decreasingincident light in the first region is compensated by the increasingincident light in the second region, so that the combined or totalcross-sectional width of incident light in both regions on thephotodiode remains at approximately A_(S). More specifically, as theangle of the incident light increases even further from the normaldirection, the first region 38 eventually disappears and the occludedregion, or the region under the mask's shadow, decreases as it moves offthe photodiode, causing the second region 40 to further increase.Accordingly, the increasing second region 40, as the mask's shadow movesoff the photodiode, nearly compensates for the cosine effect. Thus, whenthe mask's diameter is appropriately sized and spaced from thephotodiode, the photodiode's response remains nearly constant for allincident light angles except at angles near the horizon or nearlyparallel with the photodiode's surface. At the angles near the horizon,where the mask's shadow is no longer on the photodiode, the constructedocclusion effect of the mask ceases and, accordingly, thecross-sectional width of the incident light on the photodiode, and thusthe photodiode's response, is again cosine dependent (FIG. 3C).

The photodiode's reduced response for angles near the horizon iscompensated by the deflector. As shown in FIG. 4A, the deflector 16 is avertical cross-like structure formed of two generally orthogonal planarbaffles 42. The baffles may be constructed of Spectralon® or of asuitable base material, such as plastic, coated with a diffuselyreflective material, such as barium sulfate. The baffles extendgenerally perpendicular to the photodiode's surface and have a lengthsubstantially equal to the photodiode's diameter and a width or heightsubstantially equal to the distance between the mask and the aperture.As shown in FIG. 4B, the baffles allow detection of light incident atangles near the horizon by intercepting it and diffusely reflecting ittoward the photodiode and the aperture 26 (FIG. 1), or toward thephotodiode 12 and the mask 14 (FIG. 5). Preferably, the height of thebaffles is selected such that, for light incident at angles near thehorizon, the cross-sectional area of the baffles is nearly equal to theregions or areas A_(S) (FIG. 3A) for light incident at angles near thenormal direction.

In an alternative embodiment of the deflector 16, shown in FIG. 6, themask 14 is formed of a reflective material, such as Spectralon®, havinga substantial thickness and the mask's edges 44 are beveled to form thedeflector. The beveled edges provide a reflective surface that directsnearly horizontal incoming light toward the aperture or the photodiode.

The base's shoulder 24 (FIGS. 1 and 2) improves the forward sensitivityof the detector 12 and reduces its sensitivity to light incoming frombelow the horizon. The shoulder diffusely reflects light incoming fromabove the horizon and some of it reaches the photodiode, and theshoulder blocks incoming light from below the horizon that wouldotherwise reach the photodiode in the shoulder's absence.

An alternative embodiment of a light detector 10' of the presentinvention is shown in FIG. 7. In this embodiment, the dome 34 isoptional because the mask 14 and photodiode 12 are supported by thedeflector's baffles 42. The photodiode is a small pin diode (UDT: PIN040A) mounted in a small recess at the intersection of the baffles andthe mask has a diffusely reflective surface. The hemispherical cavity 22has a 1 inch radius, the mask/aperture diameter ratio is 0.9 or 90% andthe mask is spaced 0.3 inches from the aperture 26, resulting in a maskdistance to aperture diameter ratio of 0.15 or 15%. The baffles have athickness of about 3 millimeters and further have legs 46 that extend tomounting slots in the base 18. At least one baffle also has a small hole48 bored through it and its leg, and a corresponding small hole 50 isbored through the base from the appropriate slot to the rear 52 of thebase. Small wires 36 pass through the holes allowing for electricalconnection to the photodiode from the rear of the base.

The uniformity of the direction response for any of the light detector'sembodiments, 10 and 10', can be empirically optimized using a variety ofparameters. For example, the height, relative diameter, thickness, andreflectivity of the mask 14, the width and reflectivity of the shoulder24, the height and reflectivity of the deflector 16, the shape, size,and reflectivity of the cavity 22, and the photodiode's diameter, allaffect the light detector's directional response. Conversely, thedirection response can be tailored to be nonuniform, if desired, byvarying specific parameters. For example, decreasing the distancebetween the mask and the aperture will decrease the spherical sector ofthe detector's response, while increasing the detector's efficiency.Further, the light detector's spectral response can be tailored by usingspectrally selective paint on the diffusely reflective surfaces.

Although the foregoing discloses the presently preferred embodiments ofthe present invention, it is understood that those skilled in the artmay make various changes to the preferred embodiments shown anddescribed, without departing from the scope of the invention. Theinvention is defined only by the following claims.

We claim:
 1. A light detector that provides a response signal based onthe intensity of light incident from any direction within apredetermined sector, comprising:a base having a surface formed of adiffusely reflective material that defines a reflective region thatfaces the predetermined sector; a mask spaced a predetermined distancefrom the reflective region and located between the base and thepredetermined sector such that, when light is incident at an anglenormal to the base's surface, the mask occludes a portion of thereflective region; a sensor, responsive to incident light, thatgenerates the response signal based on the intensity of light that itreceives and that is located to intercept light reflections from thebase surface, wherein the base, mask and sensor are configured such thatthe light detector is substantially uniformly responsive to light fromany direction within the predetermined sector, wherein the reflectiveregion further includes a hemispherical cavity and the reflective regionis defined by the cavity's aperture and a shoulder surrounding thecavity's aperture.
 2. A light detector as defined in claim 1, whereinthe mask is circular disk having at least one flat surface and an axisthrough the flat surface's center that is aligned with an axis throughthe center of the hemispherical cavity such that the disk's flat surfaceis parallel with the cavity's aperture.
 3. A light detector thatprovides a response signal based on the intensity of light incident fromany direction within a predetermined sector, comprising:a base having asurface formed of a diffusely reflective material that defines areflective region that faces the predetermined sector; a mask spaced apredetermined distance from the reflective region and located betweenthe base and the predetermined sector such that, when light is incidentat an angle normal to the base's surface, the mask occludes a portion ofthe reflective region; a sensor, responsive to incident light, thatgenerates the response signal based on the intensity of light that itreceives and that is located to intercept light reflections from thebase surface, wherein the base, mask and sensor are configured such thatthe light detector is substantially uniformly responsive to light fromany direction within the predetermined sector, wherein the reflectiveregion further includes a hemispherical cavity and the reflective regionis defined by the cavity's aperture, and wherein the mask is a circulardisk having at least one flat surface and an axis through the flatsurface's center that is aligned with an axis through the center of thehemispherical cavity such that the disk's flat surface is parallel withthe cavity's aperture, and wherein the diameter of the disk is about 90%of the diameter of the hemispherical cavity.
 4. A light detector asdefined in claim 3, wherein the mask is spaced away from the aperture ata distance that is about 10% to 15% of the diameter of the hemisphericalcavity.
 5. A light detector that provides a response signal based on theintensity of light incident from any direction within a predeterminedsector, comprising:a base having a surface formed of a diffuselyreflective material that defines a reflective region that faces thepredetermined sector; a mask spaced a predetermined distance from thereflective region and located between the base and the predeterminedsector such that, when light is incident at an angle normal to thebase's surface, the mask occludes a portion of the reflective region; asensor, responsive to incident light, that generates the response signalbased on the intensity of light that it receives and that is located tointercept light reflections from the base surface, wherein the base,mask and sensor are configured such that the light detector issubstantially uniformly responsive to light from any direction withinthe predetermined sector, wherein the reflective region further includesa hemispherical cavity and the reflective region is defined by thecavity's aperture, and wherein the mask is a circular disk having atleast one flat surface and an axis through the flat surface's centerthat is aligned with an axis through the center of the hemisphericalcavity such that the disk's flat surface is parallel with the cavity'saperture, and further wherein the sensor is a photodiode located betweenthe mask and the reflective region, and its response signal is anelectrical signal.
 6. A light detector as defined in claim 5, whereinthe photodiode is attached to the disk's flat surface.
 7. A lightdetector as defined in claim 6, further comprising a deflector thatdirects light incident from a direction nearly parallel with theaperture toward the aperture.
 8. A light detector as defined in claim 7,wherein the light deflector is an orthogonal pair of baffles that areoriented to diffusely reflect light incident from a direction nearlyparallel with the aperture toward the aperture and the photodiode.
 9. Alight detector as defined in claim 8, wherein the disk has a thicknessthat is about 10% of the diameter of the hemispherical cavity and thedisk's edge is beveled and reflective so that is directs light incidentfrom a direction nearly parallel with the aperture toward the aperture.10. A light detector as defined in claim 1, further comprising adeflector that reflectively directs light incident from a directionnearly parallel with the aperture toward the reflective region.